1. Technical Field
This invention relates specifically to fluid absorbent polymers known as super-absorbent polymers or hydrogel forming polymer compositions. However, the invention can be used to produce novel crosslinked polymer by a novel method. Such polymers are capable of absorbing large quantities of aqueous fluids and find many applications such as absorbents in diapers to their use as thickeners in cosmetics.
2. Prior Art Background
Super-absorbent polymers are well known. They comprise a group of essentially water-insoluble, partially crosslinked hydrogel forming polymer compositions which possess the ability to absorb large quantities of aqueous fluids. Examples of such super-absorbent polymers include crosslinked homopolymers, copolymers and graft polymers of a monomer having a carboxyl group or a monomer which can be polymerized and subsequently hydrolyzed to provide carboxyl moieties in the polymer chain. Specific examples of such super-absorbent polymer include partially crosslinked polyacrylic acid polymers, polymethacrylic acid polymers, starch or polyvinyl alcohol grafted acrylic or methacrylic acid polymers and hydrolyzates of starch grafted polyacrylonitrite polymers. Specific prior art patents disclosing super-absorbent polymers include: U.S. Pat. No. 4,654,039--a crosslinked, partially neutralized polyacrylic acid polymer; U.S. Pat. No. 4,076,663--a crosslinked, partially neutralized, starch grafted polyacrylic acid polymer; U.S. Pat. No. 4,389,513--a crosslinked, partially neutralized copolymer of isobutylene and maleic anhydride; U.S. Pat. No. 4,124,748--a crosslinked, partially neutralized saponification product of a vinyl acetate/acrylic acid copolymer; and U.S. Pat. No. 3,935,099 saponified starch-polyacrylonitrile graft copolymer.
Super-absorbent polymers are generally prepared by solution and inverse emulsion/suspension polymerization processes. U.S. Pat. Nos. 4,076,663 and 4,654,039 describe well known solution polymerization methods. U.S. Pat. Nos. 4,304,706 and 4,507,438 are examples of descriptions of the well known inverse suspension emulsion polymerization procedures. The teachings of these prior art patents are hereby incorporated by reference.
Super-absorbent polymers are essentially water insoluble due to the degree of crosslinking present in the polymer. However, the degree of crosslinking not only controls water solubility but also the amount and rate of water absorption, the processability of the polymer in the manufacturing process and other performance and processability properties of the polymer. This invention is directed to an advantageous method of crosslinking super-absorbent polymers and a process for preparing said super-absorbent polymers.
Crosslinking of the polymer is normally achieved in two ways. Internal crosslinkers can be added to the polymerization reaction mixture prior to polymerization and polymerization and crosslinking effected simultaneously. Alternately, the crosslinker is added to the reaction product after polymerization has been effected and the polymer is then crosslinked, Prepolymerization addition of the crosslinker has the advantage of greater crosslinking uniformity in the polymer. However, the pre-addition method results in a higher viscosity reaction product which can result in decreased productivity, higher equipment costs and increased process energy requirements. Post-addition results in less uniformity of crosslinking in the polymer reaction product and high energy cost because it is necessary to mix the crosslinker into a viscous reaction product.
It is the object of this invention to avoid the foregoing problems and provide an improved process for preparing super-absorbent polymer compositions.